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Skyline View

Page 5.3.3 · Plain Radiography · imaging page

The patellofemoral joint seen end on, at one flexion angle out of the range the joint works through.

Pipeline status. Editorial draft. Evidence verification did not run. Not consultant reviewed. Not for clinical use.

The same topic at 4 depths

Each tier deepens the one below it and never repeats it. Choose the level you want.

Skyline View

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For Medical Students

What It Shows

A skyline view projects along the patellofemoral joint with the knee flexed, showing the patella sitting in the trochlea. 5.2.2 covers the image formation.

It shows the shape of the trochlea, the position of the patella within it and the patellofemoral joint space. 5.12.1 covers trochlear dysplasia.

It matters because it is the only plain film that shows the patellofemoral articulation directly, and it shows it at one angle. 5.12.4 covers patellar subluxation.

How It Is Acquired

Flex the knee to the protocol's angle and project along the joint, which requires the beam to clear the thigh. 5.2.5 covers magnification.

Keep the flexion angle consistent between studies, because patellar position changes through flexion. 5.12.3 covers patellar tilt.

Note that the patella engages the trochlea as flexion increases, so a flexed view is the view in which it is most likely to look normal. 5.12.4 covers patellar subluxation.

Assess both knees where instability is the question, since the comparison is informative. 5.12.1 covers trochlear dysplasia.

Record the flexion angle, because the measurements depend on it. 5.20.8 covers common reporting errors.

What Normal Looks Like

A patella centred in a trochlea with a recognisable groove and two facets of proportionate size.

A congruent joint space on both facets.

A symmetrical appearance between the two knees.

A trochlear sulcus of recognisable depth. 5.12.1 covers trochlear dysplasia.

What Abnormal Looks Like

A patella tilted or displaced laterally within the trochlea. 5.12.3 covers patellar tilt.

A shallow or flat trochlea. 5.12.1 covers trochlear dysplasia.

Lateral joint space loss with subchondral change. 5.12.7 covers patellofemoral arthritis.

Patellar osteophyte or a fracture line through a facet. 5.3.12 covers patellar fractures.

Key Learning Points

  • The view shows the patella sitting in the trochlea, end on.
  • It is the only plain film showing the patellofemoral articulation directly.
  • It shows one flexion angle out of the working range.
  • The patella engages the trochlea as flexion increases.
  • The flexion angle must be recorded because the measurements depend on it.

MRCS Level

How It Is Acquired

In a viva the expected answer names the flexion angle problem before describing any finding.

The expected points are the flexion, the consistency between studies, the engagement with flexion, the bilateral comparison, and recording the angle.

Say that instability happens near extension and this view is taken in flexion. 5.12.4 covers patellar subluxation.

Say that a normal skyline does not exclude instability. 5.12.5 covers medial patellofemoral ligament injury.

Say that trochlear shape is better assessed on cross sectional imaging. 5.12.1 covers trochlear dysplasia.

What Abnormal Looks Like

The reasoning expected is that the patella is least constrained near extension and most constrained in flexion, so a flexed view images the joint in its stable position. 3.12.2 covers the contact areas.

That makes the view good for arthritis, where the question is the articular surface, and poor for instability, where the question is what happens near extension. 5.12.7 covers patellofemoral arthritis.

Trochlear dysplasia is a three dimensional shape and the view gives one section of it. 5.12.1 covers trochlear dysplasia.

Tilt and displacement measured here change with the flexion angle, which is why the angle is part of the measurement. 5.12.3 covers patellar tilt.

How Good It Is

The skyline view is a good arthritis view and a poor instability view, and it is usually requested for instability.

Its best contribution is patellofemoral joint space and articular surface assessment. 5.12.7 covers patellofemoral arthritis.

It is limited by imaging the joint in its most stable position, and by showing a three dimensional shape in one plane. 5.12.1 covers trochlear dysplasia.

No figures appear for accuracy on this page, under decision 028, and no tilt angles, congruence angles or flexion angles appear either: those are published quantities tied to specific conventions and this environment cannot check them.

Key Learning Points

  • The patella is least constrained near extension and most in flexion.
  • The view images the joint in its stable position.
  • Good for arthritis, poor for instability.
  • Trochlear dysplasia is three dimensional and this is one section.
  • Measurements change with the flexion angle.

FRCS Level

How Good It Is

At decision level this view supports the arthritis decision and should not carry the instability one.

Isolated patellofemoral arthritis with preserved tibiofemoral compartments is a specific operative decision, and this is the view it is made on. 5.12.7 covers patellofemoral arthritis.

A flat trochlea seen here is a real finding and the degree of dysplasia needs cross sectional assessment. 5.12.1 covers trochlear dysplasia.

Instability assessment needs the measurements this view cannot give, including the tibial tuberosity position and patellar height. 5.16.4 covers the tuberosity to groove measurement.

A normal skyline in a patient with recurrent dislocation is a common and unremarkable combination. 5.12.4 covers patellar subluxation.

Where It Misleads

The commonest error is excluding instability on a normal skyline. 5.12.4 covers patellar subluxation.

The flexion angle differs between studies and the tilt appears to change. 5.12.3 covers patellar tilt.

Trochlear depth is graded from one section. 5.12.1 covers trochlear dysplasia.

Lateral facet joint space loss is read without the other compartments. 5.3.1 covers the anteroposterior view.

The view is requested where cross sectional imaging was needed. 5.1.3 covers choosing the correct investigation.

Key Learning Points

  • Instability is excluded on a normal skyline.
  • Differing flexion makes tilt appear to change.
  • Trochlear depth is graded from one section.
  • Lateral facet loss is read without the other compartments.
  • The view is requested where cross sectional imaging was needed.

Fellowship Level

Where It Misleads

At fellowship level the skyline view is a well established film whose measurements have largely been superseded and are still quoted.

Tilt and congruence angles were defined on this view and their thresholds derive from cohorts imaged at a particular flexion angle, which is rarely the angle used now. 5.21.4 covers the patellofemoral imaging literature.

The measurements that drive instability surgery are made on cross sectional imaging, so the plain film's role has narrowed to arthritis without the conventions following. 5.16.4 covers the tuberosity to groove measurement.

Dynamic patellar tracking is the question the clinic actually has, and no static view answers it. 5.18.6 covers dynamic magnetic resonance imaging.

Ultrasound and dynamic magnetic resonance imaging both address that and neither is routine, so the honest position is that the common question has no routine answer. 5.4.14 covers patellar tracking.

The useful stance is to read this view for the articular surface and to go elsewhere for position and tracking. 5.12.7 covers patellofemoral arthritis.

Key Learning Points

  • The angle thresholds derive from a flexion angle rarely used now.
  • The measurements driving instability surgery are made cross sectionally.
  • The plain film's role has narrowed without the conventions following.
  • Dynamic tracking is the clinic's question and no static view answers it.
  • Read this view for the articular surface and go elsewhere for position.

Explore Further

Where this topic came from and where it goes next in the spiral.

References

No reference list is attached to this draft. Evidence verification has not run, because source retrieval was unavailable at generation time. Under the Operations Handbook a missing reference is acceptable and an invented one is a critical failure, so nothing has been cited and no citation marker appears in the text. This page carries no diagnostic accuracy figure, which is decision 028 rather than an omission: the comparative standing of the modalities is standard teaching and a specific sensitivity written from memory is the same class of claim as an invented citation, so ACC-001 refuses the number and the page states the comparison. A reviewer with the literature can add the figures into How Good It Is without restructuring the page. Any measurement here, including a sequence parameter, an angle or a millimetre, is written from standard teaching, has not been checked, and is registered under decision 006 in pipeline/config/figures with a verification state of UNVERIFIED_FROM_MEMORY.

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